In the manufacturing of microwave integrated circuits formed on a substrate, it is often desirable to create direct connections between specific points of the circuit on the upper surface and the metallization on the lower surface. Such connections may be required for two different reasons: (1) to create a low inductance/low resistance path to ground; and (2) to provide an effective heat sinking, specifically for active devices. In the present state of the art when the substrate used is a ceramic substrate, such connections are typically obtained by cutting holes, referred to as "via holes" or "vias", through the substrate and by metallizing the wall of the holes. The holes may be cut either before or after processing the substrate. In the first case, the presence of holes significantly hinders the photolithography process, typically preventing the definition of fine details in the neighborhood of a hole. In the second case, the hole cutting process is severely limited by the need to preserve the integrity of the circuit already fabricated. This is especially true in the case of ceramic materials, e.g., alumina, where holes cannot be easily etched chemically and have to be drilled mechanically or by laser. Both processes are damaging to delicate circuit structures. In addition, the metallization of the wall alone may provide an adequate electrical connection but does not generally provide adequate heat conduction in the case of high power devices.
In the present state-of-the-art, the vias or holes are often filled with a conductive paste ("ink") of the type used in connection with thin film circuits. However, this process is only partially successful because during filling of the holes with the paste and subsequent curing, the volume of the paste is significantly reduced thereby creating voids in the filling.